This invention relates to voltage variable attenuators and more particularly to reflection type voltage variable attenuators used in microwave signal processing applications.
As is known in the art, a voltage variable attenuator is a device having an input port, an output port and a control port and provides a predetermined amount of attenuation to a signal propagating between the input port and the output port as determined by a control voltage fed to the control port. Typically, voltage variable attenuators are continuously variable over a predetermined attenuation range while providing a required operational bandwidth and impedance match. In microwave signal processing attenuator to operate from a single control voltage and to provide a linear voltage vs. attenuation response. It is also desirable to consume minimal direct current (DC) power when providing the control voltage signal. It is further desirable to provide attenuation to a signal with a minimum variation in signal phase shift.
Various types of voltage variable attenuators have been implemented using monolithic microwave integrated circuit (MMIC) technology. For example, tee type, pi type and bridge tee type attenuators have been implemented using monolithic microwave integrated circuit (MMIC) technology. One type of voltage variable attenuator implemented using MMIC technology includes a GaAs (gallium arsenide) MMIC attenuator such as a Triquint 9161 manufactured by Triquint Semiconductor, Inc. of Beaverton, Ore.
One type of a voltage variable attenuator (VVA) is known as a reflection VVA. A reflection VVA typically includes a quadrature coupler having an input port and an output port and a pair of coupled ports. A pair of matched variable termination devices are connected respectively to the pair of coupled ports. As signals are fed to the input port, the signals are attenuated by the amount of power absorbed by the pair of matched variable termination devices and the remaining signals are reflected toward the output port. An example of such a device is an attenuator having a part series no. D197 manufactured by General Microwave, Inc. of 5500 New Horizons Blvd., Amityville, N.Y. This attenuator is manufactured as a hybrid circuit on an alumina substrate requiring a plurality of PIN diodes and complex driver circuitry to provide the required current to drive the PIN diodes.
Unfortunately, known types of MMIC implemented voltage variable attenuators are non-linear and typically require two control voltages to provide a required impedance match. Furthermore, each device usually requires characterization testing to determine optimum control voltages for that device. Although various techniques for improving linearization and impedance matching have been utilized, such techniques are typically complex and require additional topography when implemented with MMIC technology. Furthermore, phase variation characteristics (i.e. at a fixed frequency, as the attenuation is varied, the amount of phase shift of a signal observed between the input port and the output port of the device) is often ignored with the above mentioned techniques.